1. Field of the Invention
This invention relates to a vacuum interrupter including an improved cylindrical coil electrode.
2. Description of the Prior Art
A vacuum interrupter for interrupting a large current includes generally a pair of main electrodes disposed in a vacuum vessel so as to be movable toward and away from each other, coil electrodes mounted on the rear surfaces of the main electrodes, and rods extending to the exterior of the vacuum vessel from the rear surfaces of the coil electrodes. Current flows from one of the rods to the other through the coil electrodes and main electrodes. When one of the rods is urged by an actuator for interrupting the current, one of the main electrodes moves away from the other main electrode, and an arc current is generated to flow across the two main electrodes. This arc current is dispersed into filamentlike arc currents and finally extinguished when an axial magnetic field produced by the current flowing through the coil electrodes is applied to the arc current.
Such a coil electrode is disclosed in, for example, U.S. Pat. No. 3,946,179. In the coil electrode disclosed in this U.S. patent, arms connected at one end thereof to a rod extend in a radial direction to be connected at the other end thereof to one end of arcuate sections respectively, and the arcuate sections extend in a circumferential direction to be electrically connected to the other end thereof to a main electrode. Thus, an arm and an associated arcuate section constitute a so-called L-shaped conductive member. Four L-shaped conductive members are mounted to the rod, and a clearance is formed between the adjacent ones of the four arcuate sections arranged in a circular pattern.
Current flows through the coil electrode via the route of the rod-arms-arcuate sections to the main electrode. Because of the presence of the clearances, the current flows through the four arcuate sections in the same direction, that is, the current flows substantially through an imaginary coil of one turn. This one-turn current produces a uniform axial magnetic field which acts to extinguish an arc current flowing across the main electrodes.
Thus, the clearances present in the known coil electrode play an important role for generation of a uniform axial magnetic field in the arcuate sections. In spite of such a great effect exhibited by the clearances, the known coil electrode is defective in that the axial magnetic field is weak in the vicinity of the clearances. Generally, an arc current has such a tendency that it migrates from a low intensity portion toward a high intensity portion of an axial magnetic field. Therefore, the arc current flowing through the portions of the main electrode near the clearances migrates toward the central area of the main electrode where the intensity of the axial magnetic field is high, and concentration of the arc current to the central area of the main electrode having the high field intensity results in localized overheating of the main electrode, thereby degrading the capability of current interruption. Since, also, the entire area of the main electrode cannot be effectively utilized for the current interruption, it becomes necessary to increase the size of the main electrode.